Review : Guilty as charged

THE first North Americans—or at least, the first who were any good at
big game hunting—arrived from Asia around 12 000 years ago. They came via
Beringia, the land bridge that sprang up between Siberia and Alaska as the sea
level fell during the last ice age. Archaeologists call them the Clovis people
after the New Mexican town where their remains first turned up.

It is now clear that they spread through both the Americas, reaching the
extreme south within 1000 years. As they went they killed virtually all the big
mammals. They hunted native herbivores to extinction. And, along with the
mammoths, mastodons and camels went the carnivores that preyed on them—the
giant running bears, a North American lion, the fearsome dire wolf and the
sabre-tooth tiger.

This is the “overkill” hypothesis, promulgated since the 1960s by Paul Martin
of the University of Arizona. But did the overkill really happen? In The
Call of Distant Mammoths, Peter Ward admirably summarises the
arguments.

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The raw statistics certainly show that something violent happened. In North
America, no fewer than 33 genera of large mammals disappeared between 12 000 and
10 000 years ago, as opposed to only 20 in the preceding 3 million years.

Some studies suggest that the demise of the large mammals was not as sudden
as Martin has suggested. Others argue that there is little direct evidence for
overkill, adding that so much killing is simply implausible because there were
so few people around. Still others argue that climate change was responsible.
Yet, Ward relates, most blame the Clovis people.

Don Grayson of Washington University is among those who argue that modern
carbon-14 dating techniques do not show a sudden die-off. True, says Ward,
but—and this is his personal contribution to the debate—modern
statistical analysis shows that mass extinctions always seem to be drawn out
even when they are instantaneous, simply because the palaeontological and
archaeological records are so patchy.

No conceivable data could show a more sudden die-off, because the extinctions
were too rapid to be captured in the physical records. Nor would we expect to
find many of the characteristic Clovis spearheads among the mammoth bones, as
some critics demand. They were much too precious for hunters to leave
behind.

Of course, the climate did change rapidly as the last ice age ended and so
did the vegetation. Against this, we have to consider that the climate has
changed rapidly at least twenty times in the past million years without any
extinctions. The animals simply migrate.

So the latest theories firmly support overkill, with a little help from
climate. Traditionally, ecologists supposed that population is determined simply
by the balance of births and deaths, and that these are constant for a given
species. Now they deal in stochastic models, adding the effects of
disasters—harsh winters, epidemics and fragmentation of habitat.

Change of climate would have broken up the mammoth feeding grounds at the end
of the ice age both in North America and Eurasia, leaving small populations that
were individually vulnerable. Models developed by Steve Mithen of Reading
University also show that hunting pressure of a mere two per cent per year would
finish the mammoths off in no time.

The full story is far more intricate and difficult to summarise. Ward tells
it well. It is also highly pertinent to our time, because present-day large
mammals are at least as precarious as the late Pleistocene mammoths. And we are
many times more destructive than the Clovis people. I have only one beef with
this book: Ward has obviously been told that popular science should be presented
as combative and diluted with travelogue. It shouldn’t. The science itself, well
told, will carry the day.